Wave filter



April 1, 1930. SHEA 1,7525?!) WAVE FILTER Filed Feb. 2'7, 1926 Patented Apr. 1, 1930 UNITED STATES PATENT OFFICE rmormz E. sane, E RUTHERFORD, NEW JERSEY, ASSIGNOR 'ro wEsrERN ELEcrErc conrm, INCORPORATED, or NEW YORK, N. Y., A CORPORATION on NEW YORK WAVE FILTER Application filed February 27, 1926. Serial No. 91,219.

This invention relates to wave filters, and

has for its objects the reduction of the cost of filter structures, and the more eflicient use of the material used in the construction of the electrical elements.

The features of the invention may be most readily described by comparing the new and improved structures embodying the invention with the well known ladder, or series-shunt,

0 t of filter described in U. S. patent to G. A.

ampbell N 0. 1,227,113 issued May 22,1917. In filters of that type there is frequently found a star combination of three capacities or a delta combination of three inductances,

. each element in the combination having a different impedance coefiicient from the and star combinations of small inductances,

thereby producing networks which are novel as to structural form; which possess'the same electrical characteristics as the corresponding ladder type structure; and which are more economlcal in the use of material.

The nature of the invention will be more fully understood by referring to the following detailed descri tion and to the accomao panying drawings, orming a part thereof, of which, Figs. 1 and 2 illustrate a general theorem on which the invention is based; Figs. 3, 4 and 5 illustrate the steps in the development of the network of Fig. 5 in which as the invention is embodied; and Figs. 6, 7 and 8 show in a corresponding manner the development of an alternative species of the invention. 2

Referring to Figs. 1 and 2, it has been shown by A. E. Kennelly in an article entitled The equivalence of triangles and threepointed stars in conducting networks published in the Electrical World and Engineer, 0 New York, Vol. XXXIY, No. 12, September 16, 1899, that a star, network of three general tained are expressed by the following equa- It is always possible to construct a network of the one type which will be equivalent to a given network of the other type at at least one selected frequency, regardless of the nature of the impedance in the given network.

Equivalence at all frequencies, however, is obtained only when the impedances of the given network are all of the same type and are related to each other in magnitude by simple numerical factors.

A delta network ofcapacities, for exam ple, can be constructed, which is at all frequencies equivalent to a given star network of capacities, and so also with regard to networks of inductances, or of complex impedances having the same resonance frequencies.

An examination of the equations shows that the total impedance of the three branches of the delta network is greater than twice the sum of the impedances of the-branches of the corres onding star network. It follows then, that t e total capacity in a delta network of condensers will be less than'half the total capacity in the corresponding star network, and that the total inductance in a star network of inductance coils will be less than half the total inductance in the corresponding form and composite electric wave filters,

tion points, of course, occur in the region of mission network to use delta networks of the series capacities C and C, are respectively capacities rather than star networks wherequal t9 the resultant capacities of the series ever possible, and similarly to use inductances combinations C C and C C The star in star connection rather than in delta. connected system C C and C in general The application of the star delta transis made up of unequal capacities, that is, it formation, in connection with wave filters, lacks symmetry of any sort whatever, this -is illustrated b the two examples shown in bein a consequence of the arbitrary manner Figs, 3 to 7. ig. 3 shows the elementary in w "ch the sections are selected. structure of a composite high-pass wave In'accordance with the invention the defilter comprising one symmetrical T section sign is carried a step farther, and the star and two half sections. connected network of capacities is replaced The symmetrical section includes two series by a delta connected network of smaller capacities C and a shunt resonant circuit ,capacities, the values of which are computed L C the half sections are of similar strucby means of equations 1; The final form of ture, but employ impedance elements of di:f-, the filter network is shown in Fig. 5. The ferent values. The sections are assumed to values 'of the capacities, CZ, 0' and (3' be all alike in the respect, that at the juncwhich constitute the delta network' are retion points each section has the same charspectively:

acteristic, or image,.impedance as the section CD022 to which it is joined, andthey are further '=E 1 assumed to have unequal propagation conp stants. 0',=- 1 (2) Formulae relating to the design of comb+022 posite wave filters may be found inan article and by O. J. Zobel, Theory and design of uni- I 0.10,,

I 6 0 022 published in the Bell System Technical Journal, Vol II, January, 1923; and the general procedure in design is described by K. S Johnson and T. E. Shea in the same journal, Vol. IV, No. '1, January, entitled Mutual inductance in wave with an introduction on filter design.

It has been found that the most practicable procedure al d outlined i f ll d A method, and p f b y the y p l l cording to this procedure the network is remethod, 9 deslgpmg flomposltii filters: lated to an equivalent network having sevbasefl HP011 conslelatwn 9 the P 913911195 eral sections of the simple series shunt type. Of slmple Symmetrlcal Sections of 01 Of The developme fbliof a related type of netof Fig. 5 may be determined by analytical ance elements are given, but the reverse proctel's ments when the transmission properties is a in a P p ass of determining the cocflicients of the ele- The transmission properties of the network methods when the coeflicients of the imped-- specifiedy is'. extremely difiicult, unless the H form. The T sections are sometimes known work in the transformation C ,7 L 7) a senes'tel'mmated 7 mld'sel'les of inductance networksis involved, 18 shown tions, and the II sections as shunt-termii Fi 6, 7 d 8, nated, or mid-shunt sect ons. 0 i The elementary composition of the network In accordance 71th h P P disclosed is-shown,'in Fig. 6, to include a symmetrical 1n the above noted articles 1t is possible to 11 t 8 section, and two half sections of simichoose a varietyof sections havin different 1, 11 f th elementary sections in attenuation characteristics and to t them tol i lik impedance coefiicients The geth'er so that the attenuation of each is connetwor i like that of Fig. 5, a hi gliass tributed to the whole, without the introducwave filt It is analogous to that of ig. tion of reflection losses at the junction points. 3, i th t h section contributes 9. fr mm In th s way the at e uatwn of a filter m y for which the attenuation is infinite. n this be built up to m ny r q l m n of l case however, the infinite attenuation is pro- Y yduced by anti-resonant circuits in series with In the specific example shown in F1 1 the line instead of by resonant circuits in the attenuation 1s characterized by t a hunt, points of infinite attenuation corresponding Design formulae for thecoefiicients of the respectively to the resonance frequencies of elements inthe elementary sections are given thethree shunt circuits. The infinite attenuain'the references hereinbefore mentioned.

Fig. 7 shows the filter design as it would attenuation below the range of free transordinarily be completed by simply merging mission. contiguous elements of like type into single Ordinarily the design is completed by units. Inductance L, correspondstothe par- 2 merging like elements of contiguous-branches allel connection of L and L in Fi 6; in-

of the component sections into single eleductance L'., to L and L and in uctance ments. In e specific instance of Fig. 3 this L, to inductance L in Fig. 6. step results in the structure of Fig. 4 in which Fig. 8 shows the network completed in accordance with the invention, a star'network of inductances, L,;, L; and L, being substi tutecl for the ,delta network constituted by inductances fl L' and L of Fig. 7. The

5 values of the star inductances are as follows:

, Two specific networks embodying the in- 15 ventionhavevbeen described, each ofwhich' possesses a. certain unity of Structure which,

in addition to the featureof economical use of materials, distinguishes it from the seriesshunt network electrically equivalent thereto.

2 'Itisto be understood,-;however, ihat the invention is not limited ufiit's Scope to the specific, embodimentsnfin eonneetion with which it has been described, but .oniiyin accordance with the appended claim;

25 What is claimed is:

A wave filter network having e air of input terminals a pairof output terminals;- and. two line paths therebetween, said network includingihree inductive impedance rpaths izarrangedi-lin-shunt} relation to said line p'gths', a delta' conneotedzgroup of threeca 1in 0' e. glln'e path each ter- (innectedrespeca p p impedances, and

5 said network bemgr'electmcelly equivalent to a; prototype wave filter :netw0rk'oomprising three filter sections of series-shunt type which are dissimilar to each other in their structure 1 and transmission characteristics.

40 In witness-whereof, hereunto subscribe my name this-26th day'of February, AQ D. it

1926. TIMOTHY E.'SHE A,' 

